(1) Field of the Invention
The present invention relates to a semiconductor laser device applicable to a blue-violet semiconductor laser device used for, for example, a light source for writing and reading of a high density optical disc, and a manufacturing method thereof.
(2) Description of the Related Art
A III-V group nitride semiconductor (hereafter referred to as InGaAlN) represented by GaN has been focused as a material which has a large band gap (3.4 eV for GaN in room temperature) and can realize a device which emits light in a wave range from green to ultraviolet light. By now, a green/blue light-emitting diode using the InGaAlN has been commercially manufactured and available for various displays, traffic signals and the like. Also, a white light-emitting diode which emits white light by exciting a phosphor using a blue light-emitting diode and an ultraviolet light-emitting diode has been commercially manufactured and used for liquid crystal back light and the like. As a field of application of the InGaAlN next to the light-emitting diode, a blue-violet semiconductor laser device which is applicable to a light source for a next-generation high density optical disc is given with expectation. It is necessary for its practical use to realize high-output and long-lived blue-violet semiconductor laser device in order to deal with high-speed writing. Through seduction of crystal defect along with development of epitaxial growth technology and higher performance along with improvement of device structure, they have reached a nearly satisfied level for the use of the next generation optical disc. From now on, it is necessary to develop a blue-violet semiconductor laser device which can satisfy an output specification and can be manufactured for mass production with high yield and with low costs.
Hereafter, it is explained about a structure of a high-output blue-violet semiconductor laser device which has been reported.
FIG. 1 is a schematic cross-section showing a structure of a conventional GaN-based blue-violet semiconductor laser device (e.g. S. Nakamura et. al., “The Blue Laser Diode”, Springer-Verlag Berlin Heidelberg N.Y.: pp. 255).
The blue-violet semiconductor laser device has an n-type GaN layer 2, an n-type AlGaN cladding layer 3, an n-type GaN guide layer 4, an InGaN multiple quantum well active layer 5, an undoped-GaN guide layer 6, a p-type AlGaN electron overflow suppression layer 7, a Ti/Au pad electrode 11, a Ti/Al/Ni/Au electrode 12, a sapphire substrate 13, a Ni/Au electrode 19, a p-type AlGaN cladding layer 20, a p-type GaN contact layer 21, and SiO2 passivation film 22.
The following layers are sequentially formed on the sapphire substrate 13 by an epitaxial growth technique using, for example, a Metal Organic Chemical Vapor Deposition (MOCVD technique) and the like: the n-type GaN layer 2; the n-type AlGaN cladding layer 3; the n-type GaN guide layer 4; the InGaN multiple-quantum well active layer 5; the undoped-GaN guide layer 6; the p-type AlGaN electron overflow suppression layer 7; the p-type AlGaN cladding layer 20; and the p-type GaN contact layer 21.
The portions of the p-type AlGaN cladding layer 20 and the p-type GaN contact layer 21 are etched so as to form a striped ridge portion. The SiO2 passivation film 22 is formed so as to cover sidewalls of the ridge portion. On the top of the ridge portion, there is an opening in the SiO2 passivation film 22, and the Ni/Au electrode 19 and the Ti/Au pad electrode 11 are formed as a p-type side ohmic electrode in the opening. Herein, a portion of the epitaxial growth layer is etched so as to expose the n-type GaN layer 2, and the Ti/Al/Ni/Au electrode 12 is formed as an n-type side ohmic electrode on the exposed n-type GaN layer 2.
In the blue-violet semiconductor laser device having the above mentioned structure, the striped ridge structure serves as a waveguide of the semiconductor laser device, light emission of 405 nm from the InGaN multiple quantum well active layer 5 is confined inside the ridge because of a difference of the refractive indexes between the p-type AlGaN cladding layer 20 and the SiO2 passivation film 22. Thus, a blue-violet laser is realized by so-called ridge waveguide structure.